Coherent radiation sources operating in the middle infrared transmission window are the subjects of ongoing research due to their usefulness in such a wide variety of applications. High peak power, high repetition rate pulsed Mid-IR lasers are in particular desired for uses such as remote sensing, chemical/pollutants detection, military systems, and nondestructive testing of materials, to name a few. It is highly desirable to have an optical system that is lightweight, compact, and requires minimum electrical input power.
Traditional paths to obtain spectral lines in the 3 to 5 micron wavelength region include gas and chemical lasers, frequency doubling of CO2 lasers, optically pumping a few semiconductor materials, and optical parametric oscillators pumped by one and two micron lasers. Chemical reaction lasers require handling of corrosive and toxic materials and this limits their practical use in a compact device and remote installations. Gas lasers excited by radio frequency sources suffer from low overall system efficiency (approximately 1%) and contribute only a single line in the band of interest. Semiconductor lasers also generate only limited spectral lines. These lasers, while simple in architecture, have to date required cryogenic cooling in order to generate moderate average powers and have been limited in their ability to generate peak power to kilowatt levels without suffering damage to their gain media.
The most efficient, compact, and versatile Mid-IR sources have been made using solid state lasers driving nonlinear optical converters. Laser transitions of the Neodymium (Nd) and Holmium (Ho) ion, at the one micron and two micron wavelength ranges respectively, have been used to pump nonlinear optical materials in various configurations. One such configuration called an optical parametric oscillator (OPO) can be used to convert their pump wavelength to two longer wavelengths in the infrared region. Note that inherent in the phase matching process of the OPO is the versatility to tune the longer wavelengths, called the signal and idler wavelength, by adjustment of crystal angle or temperature. The nonlinear process is driven by high electric field intensities. These high fields are generated typically with high peak power pulses from a Q-switched laser. In order to generate similar power levels with a continuous wave laser it must be focused much tighter, often causing unwanted thermal affects as well as limiting the interaction length by walk off.
The Neodymium laser is a more efficient continuous wave device compared to the Holmium laser in that it is a four level system and does not require cooling. It reaches threshold at low pump power levels and due to its large emission cross section, it can generate high gain, allowing narrow pulses when Q-switched. The one micron laser is at a disadvantage to a two micron laser though due to its shorter upper state lifetime. The two micron laser with its longer storage lifetime can generate much higher pulse energies for the same pump power than the one micron laser. The high pulse energies are required to drive the nonlinear process to generate Mid-IR power.
A one micron laser requires two OPO stages in order to convert most of its energy to the three to five micron range, and the extra converter reduces overall system efficiency and increases system complexity. Comparing one and two micron lasers, a single optical parametric oscillator stage is most efficiently pumped by a two micron laser since both the signal and idler waves will be located in the three to five micron Mid-IR region. In addition, the two micron laser can be used as a pump source for several nonlinear optical materials that are too absorbing when pumped by a one micron laser. By use of a material with a large non-linear coefficient, the laser line can be converted to the mid band range with good conversion efficiency. Zinc Germanium Phosphide (ZnGeP2) has near the highest Deff of all nonlinear crystals. Due to the crystal's loss near Nd based laser emission lines, the best choice for a pump laser base to generate the 3-5 micron light lies with a two micron solid state laser.
In the past several two micron lasers have been used for such a purpose as is disclosed in L. Pomeranz, P. Budni, P. Schunemann, T. Pollak, P. Ketteridge, I. Lee, and E. Chicklis, OSA TOPS ASSL, Vol. 10, pp. 259-261, 1997; and P. Budni, L. Pomeranz, M. Lemons, P. Schunemann, T. Pollak, and E. Chicklis, OSA TOPS ASSL, Vol. 19, pp. 226-229, 1998. In these lasers the Holmium ions were sensitized by co-doping with Thulium (Tm) ions. Introducing Thulium ions allows pumping with a readily available high power GaAs laser diode emitting in the range of 780 to 795 nm dependent on the host crystal. The diode pump light is highly absorbed by the Thulium ions and undergoes an efficient cross relaxation process which generates two higher energy state Thulium ions for each diode pump photon. In turn the two Thulium ions will transfer their energy to two Holmium ions, which allows high overall conversion of pump photons to two micron photons.
Original methods based on pulsed Thulium sensitized Holmium systems suffered for need of sufficient cooling requirements. The Holmium laser performance depends on temperature sensitive upconversion and Thulium to Holmium energy transfer processes. At room temperature the energy transfer is not complete and there is a reduction in the effective energy storage lifetime of Holmium. To improve the gain and allow sufficient extraction of high peak power pulses the laser crystal must be cooled.
More recent methods of resonantly pumping Holmium lasers with 1.9 micron sources circumvents the major cooling needs but adds substantial optical complexity as is described in C. Neabors, J. Ochoa, T. Fan, A. Sanchez, H. Choi, and G. Turner, IEEE J.Q.E., Vol. 31, pp. 1603-1605, 1995; and P. Budni, L. Pomeranz, C. Miller, B. Dygan, M. Lemons, and E. Chicklis, OSA TOPS ASSL, Vol. 19, pp. 204-206, 1998. If one could avoid using either Thulium sensitized Holmium or the Holmium laser to pump the optical parametric oscillator a simpler, more efficient mid-IR source could be achieved.